1. Field of the Invention
This invention relates to a liquid crystal composition for liquid crystal display elements and a liquid crystal display element using the same. More particularly, it relates to a liquid crystal composition for liquid crystal display elements of passive mode and active matrics mode, requiring a high reliability.
2. Description of the Related Art
In order that liquid crystal compositions used for conventional liquid crystal display elements exhibit positive or negative dielectric anisotropy, compounds having CN group at the terminal group or on the side chain of the molecule have often been used. Further, in order that the compositions have a broad liquid crystal phase temperature range, compounds containing an ester group in the molecule have often been used. In recent years, as the application range of liquid crystal display elements has been broadened, there have been increasing requirements for high reliability such as a high resistivity of liquid crystal materials, a low current consumption of liquid crystal elements, a high voltage-holding ratio (abbreviated to V.H.R.), etc. as well as a high contrast of display, etc., in the passive mode, active matrics mode display, and the like.
Liquid crystal materials using compounds having a group of strong polarity such as cyano group have a large contribution to the dielectric anisotropy, but the liquid crystal elements using such compounds have raised problems directed to the current consumption of the elements and hence to the display contrast. The reason has not yet been elucidated by persons of skill in the art, but it has been considered as follows:
The CN group of the terminal group or the side chain exerts a certain interaction with ionic impurities present in the display element, whereby when the compound is used for a liquid crystal element material, it has a bad influence upon the electric current value, the resistivity and the voltage-holding ratio and hence upon the contrast of display.
In order to solve these problems, liquid crystal materials composed mainly of compounds having fluorine atom in the molecule have recently been developed for exhibiting dielectric anisotropy (for example, see Japanese patent application laid-open No. Hei 2-289682).
When liquid crystal materials are used for display elements, a small quantity of a..chiral agent has usually been mixed in order to improve the alignment of liquid crystal molecules in the elements.
As such a chiral agent, for example, the following optically active compounds have been used: ##STR2##
Further, it has been reported that when two or more kinds of chiral compounds having different temperature-dependencies of the helical pitch of liquid crystal materials are blended and used as a chiral agent, it is possible to reduce the temperature-dependency of driving voltage (Japanese patent application laid-open No. Sho 63-22893).
The liquid crystal materials used for the display elements, for which the above-described high reliability and high contrast have been required, are obtained by adequately melting and blending liquid crystal compounds highly purified by means of recrystallization, column chromatography, etc., depending upon the use objects thereof, to thereby achieve the above requirements, and these liquid crystal materials usually contain chiral agents. During the preparation of these liquid crystal materials, ionic impurities are included therein. In order to remove the impurities from the liquid crystal materials, treatment with adsorbents, etc. or purification according to column chromatography are effective.
When a liquid crystal composition obtained by blending a chiral agent so far preferably used, with a fluorine-containing achiral liquid crystal compound, is purified according to treatment with an adsorbent or column chromatography, then the fluorine-containing achiral liquid crystal compound has a lower adsorptivity to the adsorbent or column packing than that of conventional chiral agent compound; hence the chiral agent is selectively adsorbed and the concentration of the chiral compound contained in the liquid crystal composition is reduced, so that the value of the helical pitch of the resulting liquid crystal material becomes longer than the desired value, resulting in an inferior display of the element.